1. Field of the Invention
This invention generally relates to systems and methods for performing measurements on an eye. More particularly, the invention relates to systems and methods for taking wavefront measurements of the eye.
2. Description of the Related Technology
One process for quantifying all the aberrations in the eye is known as wavefront analysis. Generally wavefront analysis involves illuminating an eye with a light beam, gathering light reflected from the eye and analyzing certain wavefront properties of the gathered light to determine aberrations in the eye. While an advantage of wavefront analysis is its ability to measure higher-order aberrations of the eye, measurement of the wavefront can be adversely affected in many ways, including, for example, accommodation state of the eye. When taking precise wavefront measurements of the eye, it is desirable that the subject's eye be stable, and in a natural, comfortable state, reducing or minimizing errors due to accommodation or eye movement. One way of ensuring the subject is comfortable and relaxed is to present an image to the eye which allows the subject to fixate on a specific object. When viewing this image, the subject's vision is preferably corrected to a level allowing them to fixate on the object. For example, the subject is preferably measured while viewing a natural scene at the desired distance for which the prescription will be generated. In an eye exam, this may mean viewing an eye chart or scene image placed at about sixteen feet or greater from the subject. However, a sixteen foot subject-to-object distance poses a problem for some exam areas due to space constraints.
Conventional wavefront measurement devices (examples including those available from Nidek, Tracy, and Wavefront Sciences) are monocular instruments. Some approaches to wavefront measurement employ the standard Shack-Hartmann sensor commonly used in ocular wavefront sensor devices. The Shack-Hartmann approach uses an optical element, such as a lenslet array, to divide the wavefronts from the aberrated pupil into smaller, non-overlapping sub-pupils, and forms in a common image plane the array of focused spots from all the subapertures. This approach is conceptually rooted in geometrical optics, and may suffer from well-known problems of dynamic range, limits to linearity, sub-aperture alignment issues, and increased complexity from large numbers of sub-apertures used to measure high-order aberrations beyond the common low-order Zernike modes. Another problem is that typical wavefront measurement systems require the patient to be rigidly restrained due to the length of time required for collection of the wavefront measurements. Such unnatural restraints add to the patient's discomfort and can result in increased eye movement as the discomfort increases. Additionally, using visible light for eye measurements also increases the patient's discomfort.
Another problem when determining visual acuity is that some patients, e.g., children or the elderly, may have a difficult time responding to vision tests that require the patient to make a subjective determination of which prescription produces the best vision for them. Improper responses by the patient can result in an inaccurate prescription and cause frustration in the patient and the operator administering the test. In addition, typically, wavefront systems require a skilled operator to properly position the patient and position the wavefront sensor in XYZ to get a “good” wavefront measurement. Factors which can cause erroneous results include, for example, improper XYZ positioning of the sensor, eye movement, tear film, eye blinks, eyelashes, glint, and spurious or uncontrolled accommodation. To effectively use wavefront systems and facilitate the widespread use of this technology, fewer burdens could be placed on the subjective actions of the operator and patient and more sophisticated techniques could be used to detect and control these factors. Typically, the operator must take multiple measurements and determine which measurements are valid for subsequent use. Certain methods for determining which images or processed results are similar and which outliers should be removed later could increase the effectiveness of the wavefront measurement process.
What is needed is a wavefront measurement system that overcomes one or more of the above-stated problems and other deficiencies in the art and that can be used over the widest possible patient population.